Redox amplification processes have been described, for example in British Specifications Nos. 1,268,126; 1,399,481; 1,403,418; and 1,560,572. In such processes color materials are developed to produce a silver image (which may contain only small amounts of silver) and then treated with a redox amplifying solution (or a combined developer/amplifier) to form a dye image.
The developer-amplifier solution contains a color developing agent and an oxidizing agent that will oxidize the color developing agent in the presence of the silver image which acts as a catalyst.
Oxidized color developer reacts with a color coupler to form the dye image. The amount of dye formed depends on the time of treatment or the availability of the color coupler and is less dependent on the amount of silver in the image than is the case in conventional color development processes.
Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide and compounds that provide hydrogen peroxide, e.g., addition compounds of hydrogen peroxide, cobalt(III) complexes including cobalt hexammine complexes, and periodates.
When hydrogen peroxide is used as the oxidizing agent the rate of amplification depends on the level of hydrogen peroxide in the solution and this falls with time due to reaction with the color developing agent, antioxidant (if present) and dismutation. This loss of concentration of hydrogen peroxide is a problem. For example, because in a continuous process the concentration must be made up, it introduces a risk of either over or under replenishment of the hydrogen peroxide concentration.
The present invention provides a solution to this problem by addition of boric acid or a borate (hereafter referred to collectively as borate) or other complexing compound capable of reversibly complexing with hydrogen peroxide to the developer solution. The borate or other complexing compound forms a reversible complex with the hydrogen peroxide and, as the hydrogen peroxide in the solution is used up, the complex will dissociate to provide a buffering effect, tending to stabilize the level of hydrogen peroxide in the solution and thereby reducing the decline in the rate of redox amplification.
Perborates that are complexes of borates and hydrogen peroxide and which readily dissociate in solution have been previously described in redox amplification, where they have been used as a source of hydrogen peroxide, for example in G.B. Patent Nos. 1,524,438 and 1,546,739. Sodium perborate that is usually represented by the formula NaBO.sub.3 4H.sub.2 O and is a complex of sodium metaborate (formula NaBO.sub.2) and hydrogen peroxide has been described in GB patent No. 1,546,739 as a source of hydrogen peroxide. The perborates whose use has been described have molar proportions of borate and hydrogen peroxide of approximately 1:1 and do not provide the above mentioned buffering effect.